A display sheet and display system is disclosed in a copending patent application, assigned to the same assignee as this application, U.S. Ser. No. 07/433,311 filed Nov. 8, 1989, now abandoned, entitled "Paper-Like Computer Output Display and Scanning System Therefor" which is fully incorporated herein by reference. Also incorporated herein by reference are U.S. Pat. Nos. 4,126,854 and 4,143,103 and an article entitled "The Gyricon--A Twisting Ball Display", published in the Proceedings of the S.I.D., Vol. 18/3 and 4, Third and Fourth Quarters 1977.
The display device, in sheet form, as described in the copending application comprises a thin transparent sheet having many of the attributes of paper documents. It looks like paper, has ambient light valve behavior like paper (i.e. the brighter the ambient light, the more easily it may be seen), is flexible like paper, can be carried around like paper, can be written on like paper, can be copied like paper, and has nearly the archival memory of paper. The salient features of this display material are an elastomeric host layer a few mils thick which is heavily loaded with hemispherically bichromal balls, each in the range of 5 to 200 microns in diameter. Each bichromal ball has hemispheres of contrasting colors, such as a white half and a black half, and is contained in its own spherical cavity filled with a dielectric liquid. Upon application of an electrical field between electrodes located on opposite surfaces of the host layer, the balls will rotate to present one or the other hemisphere to an observer, depending on the polarity of the field.
In the above-identified article, there is disclosed a method for fabricating bichromal balls. First, monochromatic glass balls are formed, e.g. heavily loaded with titanium dioxide so as to appear white. These are deposited in a monolayer upon a substrate. Then the balls are coated from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats only one hemisphere.
As illustrated in FIG. 1a bichromal balls 10 are loaded in liquid filled cavities 12 in a host matrix 14. Both the liquid 16 surrounding the balls and the balls themselves are dielectric. Therefore, although the balls are macroscopically electrically neutral, on a microscopic scale they have an electrical double layer comprising two layers of charges of opposite sign (as shown). One charge layer is localized at the surface of the ball and the other charge layer is in the nature of a space charge extending outward from the surface of the ball into the dielectric liquid. The measurable aspect of the electrical double layer is known as the zeta potential. The zeta potential is determined by the net surface and volume charge that lies within a shear surface associated with the motion of the ball through the liquid under an applied field. For a given liquid, the zeta potential is a function only of the ball surface material. Thus, the material properties which give rise to differences associated with the color or reflectivity of each hemisphere 18 and 20 give rise to different characteristic zeta potentials with respect to the dielectric liquid 16 in the cavity 12. It is the difference in zeta potential between the hemispheres of the ball which causes the ball to act like a dipole in the presence of an electrical field, as illustrated in FIG. 1b. The ball 10 will rotate, until its dipole vector lines up with the direction of the electrical field established between opposed electrodes 22 and 24.
In addition to the dipole charge distribution found on the bichromal ball in the presence of an electrical field there is also a monopole charge which is the net electrical charge of the entire ball. It is quite unlikely that the two hemispheres 18 and 20 having zeta potentials of opposite polarity will have the same magnitude. However, if that is the case, a monopole charge will not be established. As a result of the monopole charge, the ball 10 is caused to translate in the direction of the electrical field and will rest and be retained against the cavity wall, as illustrated in FIG. 2. In order for the ball to rotate easily in the liquid within the cavity, due to the dipole charge, it must move from contact with the cavity wall. When at rest against the cavity wall, friction and other forces will prevent it from rotating until it has been moved away once again, due to the monopole charge. It is this feature which enables long term image retention in this display device. In a copending patent application, assigned to the same assignee as this application, U.S. Ser. No. 07/784,294 filed Oct. 24, 1991, entitled "Method and Apparatus for Fabricating Bichromal Balls for a Twisting Ball Display", which is fully incorporated herein by reference, there is disclosed the flowing together of two streams of differently colored hardenable liquids into the center of a laminarly flowing host liquid to form a side-by-side bichromal stream. As the bichromal stream is transported by the host liquid as a free jet, its forward end becomes unstable and breaks up into droplets which form into spherical balls as they are moved by the host liquid. Further transport of the balls moves them past a hardening station and a separating station.
It is an object of the present invention to provide a simplified method, capable of large production rates, for forming hemispherically bichromal balls.